1. Field of the Invention
This invention relates to a coin sensor and to a method for determining the authenticity and/or denomination of a coin using a coin sensor.
2. Description of the Prior Art
It is known to make electromagnetic measurements of coins and then to process those measurements to determine whether a coin is a genuine coin belonging to a certain class or denomination. Typically, an inductance is mounted in proximity to a coin path so that the field generated by applying a drive signal to the inductance is influenced by the coin as it passes.
It is known to drive the coil using a drive signal which contains a broad spectrum of frequencies, e.g. by applying a square wave drive signal containing multiple harmonics. The influence of the coin on the field is then sampled at successive time instants relative to the transitions in the drive signal. The samples taken at different times are predominantly influenced by material at different depths within the coin. This time-domain measuring technique can have advantages as compared to frequency-domain measurements using analog filters.
U.S. Pat. No. 4,717,006 discloses a number of time-domain eddy-current measurement arrangements. The disclosure indicates that it is possible to use, as the coin sensor, either a single coil or a pair of transmit/receive coils. When using a single coil, the output voltage across the coil is measured after the drive voltage is switched off. The output voltage will decrease at a rate dependent upon the material structure of the coin. In a transmit/receive coil arrangement the coin passes in proximity to both coils, the drive signal is applied to the transmit coil, and the effects of the coin are determined by measuring the output of the receive coil.
The disclosed arrangements are unsuitable for extracting detailed measurement information relating to coins with thin plating, such as plated steel coins. In order to perform measurements which are indicative of the very thin plating material, it is necessary to take samples very close to (soon before or after) a voltage transition in the drive signal. Preferably, the measurement should include measurements taken at around two, four and eight microseconds before or after a transition. However, the disclosed circuits would not be able to achieve accurate measurements at these timings. In the single coil arrangement, if the resistance across the inductance is low, it would take a significant amount of time to discharge the energy in the coil after a transition, resulting in “ringing”, which would mask the effects of the decaying eddy currents in the coil. On the other hand, if the resistance is large, the current flow caused by the decaying eddy currents would be very small and difficult to measure. In a transmit/receive coil arrangement, the mutual inductance between the coils would mask the effects of the eddy currents in the coin.
It would be possible to use shielded, concentric coils (e.g. as shown in EP-A-489041 and EP-A-1589493) to mitigate this problem. However, this is an expensive solution which does not completely solve the problem.
It would therefore be desirable to provide an improved arrangement for time-domain eddy-current measurements of a coin which permits the taking of measurements predominantly influenced by a very thin surface layer of the coin.
It would also be desirable to extract a large number of measurements of the coin, giving very detailed information about the coin under test, and to process these measurements in order to obtain better discrimination. However, the prior art techniques for processing measurements are limited and not readily suited for use with large numbers of measurements. It would be particularly desirable to develop such a processing procedure which would be suitable for use not only with time-domain eddy-current measurements, but also with other types of measurements.